Using Gate Drivers for MOSFETs in a BLDC Motor
Scott O’Connor
Team 9
ECE480
Introduction- When building a large motor controller often MOSFET are used to control
the power to the motor. Most microcontroller operate at 5 or 3.3 voltage and can output
a small amount of current. Gate drivers are important intermediate step to provide the
Gate to source voltage and current required to turn on the mosfet. There are many
different parameter to take into consideration when designing with gate drivers. BLDC
Motors provide a unique challenge because the side gate to source voltage is floating
on each phase voltage.
Gate Driver Topology- There are many different styles and types of gate drivers
depending on your application’s power, voltage, current and switching frequency needs.
To turn on a mosfet a voltage between the source and gate is needed. (shown in figure
1). The voltage source must be large enough so that the gate source voltage is large
enough to turn on.
Figure 1
A three phase BLDC motor controller’s gate drivers work on the similar principles as a
single gate driver except that the gate to source voltage for the three high side mosfets
are floating on the phase voltage and changing independently.
Figure 2
When operating a BLDC motor controller each phase of the motor can be in three
possible states. The first state is both transistors are off; Q1 and Q4 are off. This leaves
the voltage of this phase floating and it not connected to anything. The second state is
the lower mosfet is turned off and the high mosfet is turned on: Q1 turned on and Q4
turned off. In this state, the source of the mosfet is at a high potential compared to the
source of the other two phases. The third state is the top mosfet is turned off and the
bottom mosfet is turned on: Q1 turned off and Q4 turned on. When in the third state the
phase voltage goes to zero. Since Vgs floats from the ground to the high rail when the
mosfets change from state three to state two some sort of isolation is require to supply
the gate source voltage.
Since all the lower transistors are all connected to ground their Vgs will not float.
This means that one isolated power supply can provide all the power for the gate drivers
of the lower transistors.
The high side of the MOSFETs must all have separately isolated voltage source
for Vgs. There are several ways of doing this.
Transformer Method
The transformer method uses a transformer to isolate the voltage to turn on the mosfet.
The transformer supplies the gate source voltage to the high side mosfet. In some
cases a double transformer can be used to control both the high and low mosfet.
Figure3
Figure 4
Bootstrap Method
The “Bootstrap Method” uses an IC and a capacitor. The supply voltage of the
IC charges the capacitor when the phase is low. When the phase goes high the voltage
and the charge in the capacitor is used to turn on the mosfet.
This method is great for low cost and simplicity but has a few drawbacks. The
switching frequency is limited to the time it takes to charge the capacitor. Also since the
current is shutting off quickly there is a negative voltage spike on Vs. The voltage
across the capacitor is the voltage of the IC power supply plus the voltage of this
negative spike (figure 5). This needs to be taken into account when choosing the
capacitor.
figure 5
Source/Sink CurrentChoosing a gate driver that can supply the correct amount of current to the gate
of the mosfet is critical in the design of the motor controller. The amount of current that
the gate driver can supply affects how fast the MOSFET can turn on. To understand
how much current you need one must look at the characteristic of a mosfet. A MOSFET
is a voltage controlled current source. Unlike BJT or IBJT’s the gate is controlled by
voltage. No current is consumed when the mosfet is on except extremely small amounts
of leakage current, which is negligible in power circuits . All the current to turn on the
MOSFET is to drive the gate capacitance. MOSFET’s have multiple capacitances. The
two that affect the gate driver are the gate to source capacitance and the gate to drain
capacitance. Gate to source capacitance is the capacitance seen between the source of
the transistor and the gate of the transistor. The drain to gate capacitance must also be
taken into account as it add to the miller capacitance.
To estimate the current output needed by the gate drivers the due to the
capacitance the total gate charge of the MOSFET is needed. A good estimation is to
take the total gate charge and divide it by the time required for the mosfet to turn on or
off.
800nf/1us=.8 A
This estimation is the average current to turn on the mosfet. Often the current output
given by the mosfet is the max current. This max current will only be seen during when
the capacitor is going through the miller capacitance. It is recommended to double this
number. This would mean a 1.6 amp current output would be needed.
A more accurate way of finding out the current requirement of the gate driver is to
model it as an RC circuit. The gate capacitance current is limited by the resistance of
gate and the resistance of the gate driver.
figure 6
Cross ConductanceCross Conductance happens when the top MOSFET and the bottom MOSFET
are both on at the same time. This can temporarily happen during switching where one
MOSFET doesn’t turn off all the way before the other turns on. A logic error by the
microcontroller can also lead to cross conductance. Some half bridge and three phase
gate driver IC’s have built is logic hardware to make sure that both transistors can’t be
on, even in the event of a logic error.
Gate drivers generally also have a built in delay between when one transistor
turns on and one turns off. This assures that the mosfet turning off is completely off
before the next one turns on.
Sources:
http://www.irf.com/technical-info/appnotes/an-937.pdf (figure 1)
http://ww1.microchip.com/downloads/en/AppNotes/00898a.pdf (figure 3,4.6)
http://www.fairchildsemi.com/an/AN/AN-6076.pdf (figure 5)
http://www.ferroxcube.com/news/gate%20drive%20trafo.pdf